Optical communications system have developed to a level of great sophistication and now include many elements in addition to the light source and photodetector optically coupled to each other by means of an optical fiber. Many of these elements have reflecting surfaces, and it has been found that the reflected radiation may have adverse effects on system operation. To prevent propagation of the unwanted reflected radiation, optical isolators have been developed. Optical isolators function as one way valves by permitting radiation to propagate in only one direction.
The operation of an exemplary optical isolator is easily explained. The optical isolator takes an incoming beam having arbitrary polarization and splits the incoming beam into two beams having orthogonal polarizations which will be referred to as o and e. The two polarizations take non-reciprocal paths through the optical isolator which depend on the direction of propagation of the beam. The paths in the forward propagation direction have low loss and the paths in the reverse propagation direction have high loss. The two beams, commonly referred to as the ordinary and the extraordinary beams, with orthogonal polarizations travel through the non-reciprocal portions of the optical isolator at different velocities and with consequently different path lengths. The different velocities thereby introduce some polarization mode dispersion. This dispersion is undesirable and is reduced by a polarization mode dispersion compensation element, commonly referred to by the acronym PMDCE, which reduces the polarization mode dispersion by equalizing the optical path length for both polarizations through the isolator. The PMDCE is a birefringent plate that accomplishes this equalization by presenting an optical path length which, for either beam, is the same as that of the orthogonal polarization in the non-reciprocal part of the isolator. The plate uses antireflection coatings on both major surfaces, which are parallel to each other, to reduce insertion loss. The PMDCE is placed approximately transverse to the direction of beam propagation. The entire package is often referred to as an optical isolator with the term isolator subassembly used for the non-reciprocal part of the isolator.
In a long distance optical communications system, there are typically many optical isolators. Careful measurement of the reflectivity of the optical isolators showed some residual reflectivity, and this reflectivity caused Fabry-Perot ripple in the isolator passband. There is a possibility that the ripples in individual optical isolators may line up and cause a loss at a given wavelength.